Not Applicable
Not Applicable
The present invention is directed to the operation of slag tap-type combustion apparatuses for the production of steam, generation of electric power, or for any other purpose. Application of the present invention to solid fuel-fired slag tap-type boilers will allow a reduction in noxious emissions of these boilers. More particularly, the present invention is directed to the addition of iron-bearing compounds to solid fuel-fired slag tap-type boilers for the purpose of fluxing the ash of fuels that would otherwise produce ash slag with high ash fusion temperatures and/or high viscosity, or that would otherwise be incompatible, for environmental or other reasons, with such boilers. As described in detail below, many different industrial products and by-products may be used as the iron-bearing compounds utilized in the method of the present invention. The method of the present invention may be used in heat, steam, and power production, and in any other applications of solid fuel-fired slag tap-type boilers.
The combustion of solid fuel releases energy that may be harnessed for the production of heat, steam, electricity, or for other purposes. Solid fuels presently combusted for the production of heat, steam, electricity, or other purposes include, but are not limited to, coal, biomass, petroleum coke, and other synthetic fuels. The most widely used solid fuel for electricity generation is coal. Coal is a macromolecular network composed of groups of polynuclear aromatic rings, to which are attached subordinate rings connected by oxygen, sulfur, and aliphatic bridges. Combustion of the organic matter of the macromolecular network generates heat that typically is used to generate steam that drives steam turbines and generates electricity. Along with the generation of heat, combustion of coal produces flue gases and fuel ash slag. The ash slag generally consists of the mineral matter remaining after combustion of the organic matter in the coal. Thus, the slag may include, for example, silicas, aluminas, iron oxides and other inorganic compounds.
Coal is abundant in the United States, more abundant than oil, for example, and coals from different regions of the United States and the world have different characteristics, such as chemical composition, heat value, and physical properties. Much of the coal from the eastern region of the United States has a high sulfur content and when combusted produces an ash that can be handled as a liquid at conventional combustion chamber temperatures. Conversely, low sulfur coal from the eastern region of the United States produces an ash that cannot be handled as a liquid at these temperatures, due to elevated melting temperatures. Handling low-sulfur coal from the western United States may present handling problems similar to those encountered with eastern region low sulfur coals, or they may present operational difficulties due to unfavorable slag viscosity characteristics.
Certain boiler systems that are fired with solid fuels, and more particularly fired with coal, are designed to operate with a continuous flow of liquid fuel ash or xe2x80x9cslagxe2x80x9d out of the bottom region of the boiler. Boilers of this type are variously referred to as xe2x80x9cslag tapxe2x80x9d or xe2x80x9cwet bottomxe2x80x9d boilers. The operation of these boilers is limited to coals that combustion produce an ash slag having a low ash fusion temperature (AFT), low viscosity, or other characteristic that allows the ash slag to flow from the combustion chamber of the boiler during operation of the boiler. The ash fusion temperature properties of the resultant fuel ash slag may be characterized in a number of ways, including initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT), and fluid temperature (FT). The AFT properties are commonly determined by the procedure outlined in ASTM Standard D-1857. This procedure defines the IDT as the temperature at which the first rounding of the tip of a cone formed from the ash of the coal being evaluated occurs; the ST is defined as the temperature at which the cone has fused down to a spherical lump having a height equal to the width of the base; the HT is defined as the temperature at which the cone has fused down to a hemispherical lump having a height which is one half the width of the base; and the FT is defined as the temperature at which the fused mass has spread out in a nearly flat layer with a maximum height of one sixteenth of an inch. In this specification, these temperatures are collectively referred to as the Ash Fusion Temperature or AFT characteristics of a particular ash.
Slag tap boilers fall into two general categories, both requiring production of a liquid ash slag from combustion of the solid fuel to operate properly. The first category is characterized by boilers that fire the fuel in pulverized form. Such boilers are referred to herein as xe2x80x9cpulverized fuelxe2x80x9d boilers. The second category includes boilers commonly referred to as cyclone boilers. In a cyclone boiler, crushed coal is fed into the burner end of a water cooled, horizontally oriented cylinder. Combustion air is introduced into the cylinder tangentially to impart a whirling motion to the coal. Coal fired boilers in both of these broad categories require the use of coal that produces an ash with properties characteristic of low AFT and favorable viscosity properties over the temperature range experienced in the combustion environment. Such AFT and viscosity properties are necessary to allow the ash slag to flow from the boiler. Typically, coals and other fuels with the low AFT characteristics or favorable ash viscosity properties required for use in slag tap boilers also have high sulfur concentrations. High sulfur content coals and other fuels produce greater sulfur dioxide and noxious gases when combusted in fuel-fired boilers.
Slag tap boilers are typically individually designed to bum coal from the local area. Therefore, the combustion chamber is designed to operate with the particular ash produced from a certain coal type. Conversely, coal meeting certain predetermined specifications must be used in each slag type boiler to ensure proper operation. These specifications include, but are not limited to, specifications for AFT characteristics, ash slag viscosity temperature profile range, ash slag base-to-acid ratio, as well as others. Coals which do not meet the specifications engineered into a certain designer""s slag tap boiler cannot be used in the boiler.
Phase I of the Acid Rain Program of the 1990 Clean Air Act placed a limit on the amount of sulfur dioxide (SO2) that can be emitted annually by certain sources. SO2 emissions from coal fired electric utility plants located in the eastern and midwestern states were typically limited to a greater degree than other sources. Sources of SO2 air pollution are allocated allowances, or xe2x80x9crights to pollutexe2x80x9d, based on their historic level of SO2 emissions. Each allowance permits one ton of SO2 to be released into the air and cannot be used again. At the end of each year, polluting sources must hold enough allowances to cover their SO2 emissions for that year. Any remaining allowances can be sold, traded, or banked for future use. If a source of air pollution does not have enough allowances to cover its SO2 emissions, it can buy allowances on the open market. Every year the limit on the amount of SO2 that can be emitted annually is reduced. Allowances are, therefore, a valuable commodity and emissions of SO2 are becoming more expensive every year. Phase II of the Acid Rain Program, which began in the year 2000, further tightens the annual SO2 emission limits imposed on coal fired utilities and on smaller, cleaner burning plants.
Therefore, burning of high sulfur coals has become or will shortly become cost prohibitive for many electric or steam generation facilities. Since slag tap or cyclone boilers were designed to operate using high sulfur coals with low AFT characteristics, continued operation of these facilities using high sulfur coals and conventional power production methods will continue to become increasingly costly, to producers and consumers alike. Lower sulfur coals generally have ash chemistry and AFT characteristics that are outside the range for which these boilers were originally designed and, therefore, use of these alternate fuels can cause significant operational problems. For example, lower sulfur coals typically have higher AFT characteristics (i.e., higher IDT, ST, HT, and FT) and, therefore, the resultant fuel ash may not flow from the bottom of the boiler, a requisite with slag tap boilers.
In order to address the problems associated with the use of alternate coal types in slag type boilers, attempts have been made to modify the AFT characteristics of coals. For example, U.S. Pat. No. 5,364,421 describes a process wherein bituminous coal, which is unsuitable for use in slag tap boilers, is blended with lignitic coal. The blend is adjusted such that the resultant fuel ash has a viscosity at or below 250 poise at 2600xc2x0 F., the operating temperature of the combustion chamber. The ""429 patent states that coals having ash viscosity in this range are suitable for use in slag tap boilers. However, the technique of blending coals to adjust AFT characteristics has major limitations because the amount of high sulfur coal that can be blended with low sulfur coal and still operate with the sulfur emissions regulations is limited. Also, the blending of coals has only a limited effect on the properties of the ash slag. Therefore, this method has limited industrial applicability.
Thus, there exists a need for a method of modifying the operation of solid fuel-fired slag tap boilers to allow the boilers to operate with alternate fuels for which the boilers were not designed. There also exists a need for a method of altering the AFT characteristics of low sulfur coals to render them suitable for use in certain slag tap boilers.
In addition, there exists a need for a method of maintaining stable boiler operation when operating slag tap boilers with solid fuels that would produce ash slag with AFT characteristics outside the range for which the boilers were originally designed.
The present invention provides a method of operating a solid fuel fired boiler comprising introducing a solid fuel and an iron-bearing material into the boiler. The solid fuel is at least partially combusted in the boiler to produce an ash slag, wherein the ash fusion temperature characteristics (i.e., one or more of the IDT, ST, HT, and FT) of the ash slag are different than the ash fusion temperature characteristics of the ash slag that would result on combustion of the solid fuel alone. The method of the present invention is particularly applicable to slag tap boilers, including cyclone-type boilers. These boilers are, typically, designed to operate with a liquid ash slag.
The iron-bearing material may be, but is not limited to, at least one of iron ore beneficiation tailings, iron ore fines, pelletized blends of coal and iron-bearing material, pelletized solid fuel containing iron-bearing compounds, iron-bearing boiler ash, mill scale from steel production, dust from blast furnace gas cleaning equipment, flue dust from sinter plants, and other materials including iron or including material that bears iron. The iron-bearing material may be blended with the coal or other solid fuel that is to be fired in the boiler prior to or during combustion to modify one or more properties of the ash slag to meet the design specifications of the boiler. These properties may be, but are not limited to, ash fusion temperature characteristics, ash slag viscosity, ash slag base-to-acid ratio, as well as other properties.
According to the present invention, the iron-bearing material may be fed into the slag tap or other solid fuel fired boiler in one or more of several different locations. In pulverized fuel slag tap boilers, the material bearing iron compounds may be fed into, for example, the fuel pulverizers, the fuel transfer system, directly into the boilers, into the combustion chamber enclosure or any other location in the system. In cyclone slag tap boilers, the iron-bearing material may be fed into, for example, the fuel storage bunker, the fuel transfer system, the cyclone boilers directly, the combustion enclosure, or any other location in the system.
The method of the invention allows lower sulfur coals, as well as other fuels typically unsuited for use in slag top boilers, to be used in such boilers, significantly reducing SO2 emissions.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend such additional details and advantages of the present invention upon making and/or using the stainless steels of the present invention.